Cysteine Inhibits Mercury Methylation by Geobacter sulfurreducens PCA Mutant ΔomcBESTZ

摘要

Cysteine enhances Hg uptake and methylation by the Geobacter sulf urreducens PCA wild-type (WT) strain in short-term assays. The prevalence of this enhancement in other strains remains poorly understood. We examined the influence of cysteine concentration on time-dependent Hg(II) reduction, sorption, and methylation by PCA WT and its c-type cytochrome-deficient mutant (ΔomcBESTZ) in phosphatebuffered saline. Without cysteine, the mutant methylated twice as much Hg(II) as the PCA WT, whereas addition of cysteine inhibited Hg methylation, regardless of the reaction time. PCA WT, however, exhibited both time-dependent and cysteine concentration-dependent methylation. In a 144 h assay, nearly complete sorption of the Hg(II) by PCA WT occurred in the presence of 1 mM cysteine, resulting in our highest observed level of methylmercury production. The chemical speciation modeling and experimental data suggest that uncharged Hg(II) species are more readily taken up and that this uptake is kinetically limiting, thereby affecting Hg methylation by both the mutant and WT.